Article for high temperature service

ABSTRACT

An article for high temperature service is presented herein. One embodiment is an article including a substrate having a silicon-bearing ceramic matrix composite; and a layer disposed over the substrate, wherein the layer includes silicon and a dopant, the dopant including aluminum. In another embodiment, the article includes a ceramic matrix composite substrate, wherein the composite includes a silicon-bearing ceramic and a dopant, the dopant including aluminum; a bond coat disposed over the substrate, where the bond coat includes elemental silicon, a silicon alloy, a silicide, or combinations including any of the aforementioned; and a coating disposed over the bond coat, the coating including a silicate (such as an aluminosilicate or rare earth silicate), yttria-stabilized zirconia, or a combination including any of the aforementioned.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a national stage application under 35 U.S.C.§371(c) of prior filed PCT application serial number PCT/US15/045593,filed on Aug. 18, 2015, which claims the benefit of U.S. ProvisionalApplication No. 62/041,184, filed Aug. 25, 2014. The above-listedapplications are herein incorporated by reference.

BACKGROUND

The present technology generally relates to high temperature machinecomponents, such as components used in gas turbine assemblies, and tomethods for protecting machine components from exposure to hightemperature environments.

Ceramic materials containing silicon and metal alloys containing siliconhave been proposed for structures used in high temperature applicationsas, for example, gas turbine engines, heat exchangers, internalcombustion engines, and the like. Ceramic matrix composites (CMCs) thatcontain silicon carbide, such as composites including a silicon carbidematrix and silicon carbide reinforcement, have been used for these hightemperature applications. However, the environments characteristic ofthese applications often contain reactive species, such as water vapor,which at high temperatures may cause significant degradation of thematerial structure. For example, water vapor has been shown to causesignificant surface recession/thickness loss and mass loss insilicon-bearing materials. The water vapor reacts with the structuralmaterial at high temperatures to form volatile silicon-containingspecies, often resulting in unacceptably high recession rates.

Environmental barrier coating (EBC) systems are applied tosilicon-bearing materials and other material susceptible to attack byreactive species, such as high temperature water vapor. EBC systemsprovide protection by prohibiting contact between the environment andthe surface of the material. EBC systems applied to silicon-bearingmaterials, for example, are designed to be relatively stable chemicallyin high-temperature, water vapor-containing environments. One EBCsystem, as described in U.S. Pat. No. 6,410,148, comprises a silicon orsilicon-containing bond layer (also referred to herein as a “bond coat”)applied to a silicon-bearing substrate; an intermediate layer comprisingmullite or a mullite-alkaline earth aluminosilicate mixture depositedover the bond layer; and a top layer comprising an alkaline earthaluminosilicate deposited over the intermediate layer. In anotherexample, U.S. Pat. No. 6,296,941, the top layer is a yttrium silicatelayer rather than an aluminosilicate.

Although turbine components and other articles that include suchprotective systems as described above have provided good performance,the prevailing trend towards harsher service conditions, including, forinstance, increased operating temperatures for improved turbineefficiency, creates a continuing need in the industry for componentswith even higher temperature capability, along with methods forfabricating such articles.

BRIEF DESCRIPTION

Embodiments of the present invention are provided to meet this and otherneeds. One embodiment is an article comprising: a substrate comprising asilicon-bearing ceramic matrix composite; and a layer disposed over thesubstrate, wherein the layer comprises silicon and a dopant, the dopantcomprising aluminum.

Another embodiment is an article comprising: a substrate comprising asilicon-bearing ceramic matrix composite; a bond coat disposed over thesubstrate, the bondcoat comprising elemental silicon, a silicon alloy, asilicide, or a combination including any of the aforementioned, andfurther comprising aluminum oxide at a concentration such that a ratioof aluminum atoms to the sum of aluminum atoms plus silicon atoms[Al/(Al+Si)] in the bond coat is in a range from 0.01 to 0.15; and atopcoat disposed over the bondcoat, the topcoat comprising a silicate(such as an aluminosilicate or rare earth silicate), yttria-stabilizedzirconia, or a combination including any of the aforementioned.

Another embodiment is an article comprising: a substrate comprising asilicon-bearing ceramic matrix composite; a bondcoat disposed over thesubstrate, the bondcoat comprising silicon, a silicon alloy, a silicide,or combinations including any of the aforementioned, and optionallyfurther comprising a dopant comprising aluminum oxide; an over-coatdisposed over the bondcoat, the over-coat comprising silica and a dopantcomprising aluminum oxide; and a topcoat disposed over the over-coat,the topcoat comprising a silicate (such as an aluminosilicate or rareearth silicate), yttria-stabilized zirconia, or a combination includingany of the aforementioned.

Another embodiment is an article comprising: a ceramic matrix compositesubstrate, wherein the composite comprises a silicon-bearing ceramic anda dopant, the dopant comprising aluminum; a bond coat disposed over thesubstrate, where the bond coat includes elemental silicon, a siliconalloy, a silicide, or combinations including any of the aforementioned;and a coating disposed over the bond coat, the coating comprising asilicate (such as an aluminosilicate or rare earth silicate),yttria-stabilized zirconia, or a combination including any of theaforementioned.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawing in whichlike characters represent like parts, wherein:

FIG. 1 illustrates a cross section of an article in accordance with anembodiment of the present invention.

FIG. 2 illustrates a cross section of an article in accordance withanother embodiment of the present invention.

FIG. 3 illustrates a cross section of an article in accordance withanother embodiment of the present invention.

DETAILED DESCRIPTION

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about”, and “substantially” is not to be limited tothe precise value specified. In some instances, the approximatinglanguage may correspond to the precision of an instrument for measuringthe value. Here and throughout the specification and claims, rangelimitations may be combined and/or interchanged; such ranges areidentified and include all the sub-ranges contained therein unlesscontext or language indicates otherwise.

In the following specification and the claims, the singular forms “a”,“an” and “the” include plural referents unless the context clearlydictates otherwise. As used herein, the term “or” is not meant to beexclusive and refers to at least one of the referenced components beingpresent and includes instances in which a combination of the referencedcomponents may be present, unless the context clearly dictatesotherwise.

As used herein, the terms “may” and “may be” indicate a possibility ofan occurrence within a set of circumstances; a possession of a specifiedproperty, characteristic or function; and/or qualify another verb byexpressing one or more of an ability, capability, or possibilityassociated with the qualified verb. Accordingly, usage of “may” and “maybe” indicates that a modified term is apparently appropriate, capable,or suitable for an indicated capacity, function, or usage, while takinginto account that in some circumstances, the modified term may sometimesnot be appropriate, capable, or suitable.

In particular aspects of the present disclosure, the term “hightemperature” refers to a range of between about 2200° F. and about 3000°F. (about 1200° C. and about 1650° C.).

The terms “silicon-containing” and “silicon-bearing” are usedinterchangeably herein to mean any material that includes, but is notlimited to, silicon. Examples of such materials include withoutlimitation elemental silicon, alloys and solid solutions that includesilicon as a component, and compounds that include silicon. Similarly,the terms “aluminum-containing” and “aluminum-bearing” are usedinterchangeably herein to mean any material that includes, but is notlimited to, aluminum. Examples of such materials include withoutlimitation elemental aluminum, alloys and solid solutions includingaluminum as a component, and compounds that include aluminum.

FIG. 1 depicts an article 100 illustrative of embodiments of the presentinvention. The article 100 includes one or more protective layers, suchas layers 104, 106, and 110, disposed on a substrate 102. The one ormore protective layers may be referred to collectively herein as an“environmental barrier coating system” or “EBC system.” In oneembodiment, the nature of article 100 may be as described in, forexample, U.S. 2011/0052925 A1, any of the references discussed above, orother references describing coatings for protection of silicon-bearingarticles at high temperatures. For illustrative purposes, article 100 insome embodiments includes a sealing layer 110, such as a layer includingan alkaline-earth aluminosilicate, disposed over a substrate 102 of thearticle 100. The substrate 102 may be made from any suitable material,such as a ceramic or an intermetallic material. The substrate maycomprise a ceramic, for example an oxide, nitride, or carbide. Thesubstrate 102 may include a silicon-containing material, such as siliconnitride, molybdenum disilicide, or silicon carbide. This material may bea ceramic-matrix composite (CMC) material, such as a material made of amatrix phase and a reinforcement phase. The matrix phase, thereinforcement phase, or both of these phases may comprise asilicon-bearing ceramic, such as silicon carbide (SiC) or siliconnitride; composites including such materials are referred to herein as“silicon-bearing ceramic matrix composites.” The article 100 may be acomponent of a gas turbine assembly, such as, for example, a combustionliner, transition piece, shroud, vane, or blade.

A bond coat 104 may be disposed over the substrate 102, and in someembodiments, such as the one illustrated in FIG. 1, bond coat 104 isdisposed directly on substrate 102. Other layers, such as the sealinglayer 110, if present, may be disposed over bond coat 104. The bond coat104, in some embodiments, comprises silicon; for example, bond coat 104may include a silicon alloy or elemental silicon, as described in, forexample, U.S. Pat. No. 6,299,988, or a silicide, for example asdescribed in U.S. Patent Application Publication No. US 20110097589 A1.The bond coat 104 may be used, for example, to mitigate thermal stressesor to inhibit chemical reactions between the substrate 102 and otherlayers, such as the sealing layer 110. The bond coat 104 may also beused as an oxygen barrier to prevent oxygen from chemically interactingwith substrate 102.

Where the bondcoat 104 includes elemental silicon or silicon-containingmaterial, an intermediate layer (not shown) may be disposed between thesealing layer 110 and bondcoat 104. The intermediate layer is made of abarrier material that is substantially inert with respect to siliconoxide to promote chemical stability in the EBC system. “Substantiallyinert” means that there is at most only incidental interaction(solubility or reactivity) between silica and the barrier material. Rareearth disilicates, such as disilicates of yttrium, ytterbium, lutetium,scandium, and other rare earth elements, are non-limiting examples ofsuitable barrier materials.

A topcoat 106, typically including some form of oxide material, may bedisposed to provide thermal insulation (a thermal barrier coating),environmental protection (an environmental barrier coating), or acombination of these functions. The selection of a suitable topcoatmaterial will depend on the type of environment the article is to beexposed to, the composition of the underlying coatings and substrate,the cost of processing, and other factors. The topcoat 106 may be aceramic material including, but not limited to, a silicate (such as analuminosilicate or rare earth silicate), and yttria-stabilized zirconia.The topcoat 106 may contain a rare earth monosilicate and/or rare earthdisilicate. The topcoat 106 may be a dual-layer coating, with an outerlayer of rare earth monosilicate and an inner layer of rare earthdisilicate. The rare earth elements associated with these monosilicateand disilicate materials may include one or more of yttrium, ytterbium,lutetium, and scandium. The outer layer may be yttrium monosilicate andthe inner layer may be a rare earth disilicate (such as yttriumdisilicate, for example).

The thickness of any of the various coating layers described above maybe chosen to provide adequate protection for a given service time whilekeeping thermal stresses to a sustainable level. Moreover, coatingthickness may also be determined by the ability of a selected coatingmethod to produce a continuous layer over the deposition area.Non-limiting examples of approximate thickness ranges for the variouscoatings include the following: for the sealing layer 110, from about 25μm to about 150 μm; for the bondcoat 104, from about 75 μm to about 125μm; for the intermediate layer, from about 50 μm to about 100 μm; forthe topcoat 106, from about 50 μm to about 500 μm. For the dual-layertopcoat described above, the yttrium monosilcate outer layer may be fromabout 25 μm to about 50 μm. The coatings described above can bedeposited using coating technology including, for example, but notlimited to, plasma spray technology, chemical vapor deposition, andslurry-based coating processes; such techniques and their application todepositing coatings described herein will be apparent to one of ordinaryskill in the art.

In an article 100 that includes an EBC system, such as illustrated inFIGS. 1 and 2 herein, silicon-bearing components of the article, suchas, for example, a silicon-bearing bond coat 104 and silicon carbideconstituents of a CMC substrate 102, rely on protection against hightemperature oxidation by formation of a dense silica film, which formson exposure to oxidizing conditions. One of the limitations ofconventional EBC systems is that, on exposure to high temperatures inoxidizing conditions, the silica film crystallizes. The crystallizationcan increase the oxidation rate of the underlying silicon-bearingmaterial. Crystalline silica (crystobalite) goes through phase changeson cooling from an oxidation temperature of over approximately 2200degrees Fahrenheit (approximately 1200° C.), and the crystallographicchanges associated with the phase changes can induce cracking andspallation of the silica film, accelerating the oxidation of silicon anddegrading adhesion of outer coating layers to the underlying CMCsubstrate.

Ceramic matrix composites used for gas turbine applications ofteninclude a silicon carbide fiber and a silicon carbide matrix, and arehence called “SiC/SiC” composites. These composites can be made in awide variety of ways including by Chemical Vapor Infiltration (CVI) andby silicon Melt Infiltration (MI). Composites made by melt infiltrationoften also contain unreacted silicon in the matrix in addition tosilicon carbide. Normally, composite substrates would not be directlyexposed to oxidizing conditions unless the EBC detaches from thesubstrate. Silicon carbide exposed to high temperature, oxidizingconditions forms a protective silica film in the same fashion aselemental silicon. However, the oxidation of silicon carbide, such as atan exposed portion of CMC substrate or in a bond coat that includessilicon carbide, produces not only silica, but also carbon monoxide(CO). The carbon monoxide can diffuse out thru an amorphous silica filmwithout causing substantially detrimental effects. However, it isunlikely that CO gas can readily diffuse thru a crystalline silicacompound. Consequently, CO pressure can build up at the SiC/crystallinesilica interface, causing the crystalline silica film to crack, furtherincreasing the oxidation rate of underlying SiC. Thus, thecrystallization can have even a more detrimental effect on the oxidationof silicon carbide than on that of silicon. Similar problems exist forother silicon-containing compounds that form gases on oxidation, such assilicon nitride.

To address this problem, article 100 in accordance with embodiments ofthe present invention further includes, in addition to any of thematerials described above, an aluminum-containing dopant disposed toform and/or sustain an aluminum-containing silica film on exposure ofarticle 100 to a high temperature, oxidative environment. Thealuminum-containing silica film is more resistant to crystallizationwhen exposed to an oxidizing atmosphere at high temperature than a filmthat does not contain aluminum-bearing dopant. The dopant is disposed inone or more portions of article 100 that may be susceptible tooxidation, such as bond coat 104, substrate 102, or both. As noted infurther detail, below, the dopant additionally or alternatively may bedisposed in a layer, such as a silica layer, over the substrate or overthe bondcoat.

In one embodiment, article 100 includes substrate 102 and a layer, suchas layer 104, disposed over substrate 102, and it is layer 104 thatincludes silicon and the dopant. In some embodiments, thissilicon-bearing layer 104 is disposed directly on substrate 102.Examples of such embodiments include those where layer 104 is a bondcoat in an EBC system, as described previously. A particular example isone in which layer 104 includes elemental silicon, although, as notedpreviously, silicon alloys and silicides are also examples of acceptablealternative materials for use in bond coat 104. In other embodiments, adoped silica-bearing layer is disposed as an over-coat 105 (FIG. 2) overa silicon-bearing bond coat 104 that is disposed on substrate 102. In analternative embodiment, the doped silica layer may be directly disposedon substrate 102.

Substrate 102 comprises a silicon-bearing ceramic matrix composite, suchas a composite that includes silicon carbide, silicon nitride, or acombination that includes one or both of these. As noted above, aSiC—SiC ceramic matrix composite is one example of a material for use insubstrate 102.

Suitable aluminum-bearing dopants for use in the embodiments describedherein include, without limitation, elemental aluminum, aluminum oxide,aluminum carbide, aluminum nitride, aluminum boride, and mixturesthereof. In certain embodiments, aluminum oxide is included as a dopantbecause it does not form any gaseous compound on oxidation. In someembodiments, aluminum oxide is present in layer 104 in the form ofnanoparticles, meaning a plurality of particles having a median longestdimension less than about 1000 nanometers. In some embodiments, themedian longest dimension of the nanoparticles is less than about 200nanometers. The use of small-diameter particles, such as nanoparticles,for instance, may enhance the ability of the aluminum oxide to dissolveor otherwise disperse well within the silica film that forms duringservice in high temperature, oxidizing environments. Nanoparticles maybe added to silicon-based coatings, for example, by applying techniquessuch as liquid-injection plasma spray, in which a slurry of siliconpowder, dopant powder nanoparticles, and a liquid carrier is fed to aplasma torch and deposited as a doped, silicon-bearing coating. Othertechniques, such as high-velocity oxyfuel deposition of mixed powderfeedstock may be applied as well.

The amount of dopant present in the doped layer, such as layer 104, isselected to be sufficiently high to usefully reduce the rate ofcrystallization of silica at temperatures in the “high temperature”range noted previously relative to an undoped silica layer. On the otherhand, the presence of the dopant may increase the oxygen transport ratethrough the silica film, and so the amount of dopant present is selectedbased on balancing these competing effects. In one embodiment, theamount of dopant is selected to be sufficiently low to avoid increasingthe oxygen transport through the layer by more than a factor of about10. For instance, in some embodiments the amount of dopant, such asaluminum oxide, is selected such that a ratio of aluminum atoms to thesum of aluminum atoms plus silicon atoms [Al/(Al+Si)] is in a range from0.01 to 0.15. In certain embodiments, this ratio is about 0.01 to 0.10,which means the aluminum oxide level in a resulting silica film would befrom about 0.5 to 5 mole percent. It is typically desirable to keep thealuminum level as low as possible but sufficiently high to reduce thesilica crystallization to an acceptable level.

As noted previously, some embodiments of article 100 include topcoat106. Any of the various coating architectures and candidate materialsdescribed above is suitable for use in embodiments employing analuminum-doped, silicon-bearing layer 104. Particular examples ofsuitable oxides often employed as topcoat 106 include without limitationa silicate (such as an aluminosilicate or a rare earth silicate),yttria-stabilized zirconia, and combinations including any of these.

To further illustrate the concepts described above, one particularembodiment of article 100 includes a substrate 102 comprising asilicon-bearing ceramic-matrix composite, such as a SiC/SiC composite. Abond coat 104 is disposed over substrate 102, and this bond coat 104includes elemental silicon, a silicon alloy, a silicide, or combinationsthat include one or more of these. Bond coat 104 further includesaluminum oxide as a dopant, at a concentration such that a ratio ofaluminum atoms to the sum of aluminum atoms plus silicon atoms[Al/(Al+Si)] in the bond coat is in a range from 0.01 to 0.15. Topcoat106 is disposed over bondcoat 104, with or without one or moreintervening layers as noted previously, and topcoat 106 includes asilicate (such as an aluminosilicate or rare earth silicate),yttria-stabilized zirconia, or a combination including one or more ofthese.

As a further illustration, referring to FIG. 2, another particularembodiment of article 100 includes a substrate 102 comprising asilicon-bearing ceramic matrix composite, such as a SiC/SiC composite. Abond coat 104 is disposed over substrate 102, and this bond coat 104includes elemental silicon, a silicon alloy, a silicide, or combinationsthat include one or more of these. Bond coat 104, in some embodiments,includes a dopant such as aluminum oxide as described above, while inother embodiments the bond coat 104 is not doped. An over-coat 105 isdisposed over the bondcoat 104, the over-coat 105 comprising silica anda dopant comprising aluminum oxide. Topcoat 106 is disposed overover-coat 105, with or without one or more intervening layers as notedpreviously, and topcoat 106 includes a silicate, for example, analuminosilicate or rare earth silicate; yttria-stabilized zirconia, or acombination including one or more of these.

In some embodiments, instead of or in addition to having dopant presentin a layer disposed on a substrate, the dopant is disposed within thesubstrate itself. Referring to FIG. 3, in an embodiment illustrative ofthis aspect, article 100 includes a silicon-bearing ceramic matrixcomposite 102, such as a composite including silicon carbide, siliconnitride, or combinations including at least one of these. The composite102 includes an aluminum-bearing dopant, which serves to incorporatealuminum into a silica film that forms on composite 102 should it becomeexposed to a high temperature, oxidative environment. The incorporationof aluminum from the doped composite 102 into the silica film helps tostabilize the amorphous form of the oxide, reducing the rate of silicacrystallization at temperatures above about 2200 degrees Fahrenheit andthereby helping to prolong the useful life of article 100 as discussedpreviously.

In some embodiments, the dopant is present at least in a doped region130 of composite 102; doped region 130 is typically inclusive of asurface 132 of article and extends at least about 25 micrometers(beneath surface 132. In other embodiments, the dopant is distributedsubstantially uniformly within the entirety of composite 102, and thusin this instance doped region 130 is coincident with the volume ofcomposite 102. In certain embodiments, regardless of how the dopant isdistributed within composite 102, the dopant is present in doped region130 in a concentration range similar to that described above for dopedcoating layers; that is, the aluminum concentration within doped region130 is selected such that a ratio of aluminum atoms to the sum ofaluminum atoms plus silicon atoms [Al/(Al+Si)] within doped region 130is in a range from 0.01 to 0.15. Similarly to what was describedpreviously, the concentration of dopant in substrate 102 is typicallyselected to usefully reduce the rate of crystallization of silica attemperatures above 2200 degrees Fahrenheit, relative to an undopedsilica layer, without increasing the oxygen transport through the layerby more than a factor of about 10.

Referring still to FIG. 3, article 100 often includes at least onecoating 135 disposed over composite 102. Coating 135 may be part of anEBC system of a type commonly employed in the art to protectsilicon-bearing ceramic composite materials from high temperature,oxidative environments, with possible coating architectures andmaterials for various layers as noted previously. As shown in FIG. 3, incertain embodiments article 100 includes a bond coat 104 disposed overthe doped composite 102, and a topcoat 106 disposed over bond coat 104.In some embodiments, bond coat 104 also includes an aluminum-bearingdopant as described previously. Bond coat 104 may include silicon, suchas elemental silicon, silicon alloys, or silicides; topcoat 106 mayinclude one or more oxides such as a silicate (for instance, analuminosilicate or rare earth silicate), or yttria stabilized zirconia.As described previously for common EBC systems generally, one or moreintervening layers may be disposed between bond coat 104 and top coat106.

To further illustrate embodiments of this type, referring to FIG. 3, anarticle 100 includes a ceramic matrix composite substrate 102, whereinthe composite comprises a silicon-bearing ceramic and a dopant, thedopant comprising aluminum; a bond coat 104 disposed over substrate 102,where the bond coat 104 includes elemental silicon, a silicon alloy, asilicide, or combinations including one or more of these; and a topcoat106 disposed over bond coat 104, the top coat 106 comprising a asilicate (such as an aluminosilicate or rare earth silicate),yttria-stabilized zirconia, or a combination including any of theaforementioned. In some embodiments, bond coat 104 further includes adopant comprising aluminum oxide.

Example

The following example is presented to further illustrate non-limitingembodiments of the present invention.

Substrates made of silicon carbide were coated with an EBC system thatincluded a silicon-bearing bond coat. In a baseline sample, the bondcoat did not include a dopant, while a test sample included about 10percent by weight aluminum oxide [an Al/(Al+Si) ratio of about 0.06].The bond coats were deposited by plasma spray; in the case of the dopedbond coat, the alumina was added to the coating by feeding a mixture ofsilicon powder and aluminum oxide powder (nominal median diameter 15micrometers) to a plasma spray torch and co-depositing the materials.The samples were exposed to an oxygen-bearing environment at about 1400degrees Celsius (2550 degrees Fahrenheit) for 1500 hours and then thebond coat was examined. In the baseline sample, the undoped bond coatwas completely crystallized, while in the test sample, the doped bondcoat exhibited two distinct regions: a crystallized region and anamorphous region. This result suggests aluminum doping helps toforestall the crystallization of the bond coat at extended exposure toelevated temperature in an oxidizing environment, relative to the rateshown for undoped material.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

What is claimed is:
 1. An article comprising: a substrate comprising asilicon-bearing ceramic matrix composite; and a layer disposed over thesubstrate, wherein the layer comprises silicon and a dopant, the dopantcomprising aluminum.
 2. The article of claim 1, wherein the articlefurther comprises a gas turbine combustion liner, a gas turbinetransition piece, a gas turbine shroud, a gas turbine vane, or a gasturbine blade.
 3. The article of claim 1, wherein the silicon-bearingceramic matrix composite comprises silicon carbide, silicon nitride, ora combination including any of the aforementioned.
 4. The article ofclaim 1, wherein the layer is disposed directly on the substrate.
 5. Thearticle of claim 1, wherein the layer comprises elemental silicon, asilicon alloy, a silicide, or a combination including any of theaforementioned.
 6. The article of claim 1, wherein the layer compriseselemental silicon.
 7. The article of claim 1, wherein the layercomprises silica.
 8. The article of claim 7, wherein the layercomprising silica is disposed over a bond coat, the bond coat disposedover the substrate.
 9. The article of claim 8, wherein the bond coatcomprises elemental silicon, a silicon alloy, a silicide, or acombination including any of the aforementioned.
 10. The article ofclaim 1, wherein the dopant is present in the layer in an amounteffective to reduce the crystallization rate of silica formed on orpresent in the layer without increasing the rate of oxygen transportthrough the layer by more than a factor of about
 10. 11. The article ofclaim 1, wherein aluminum is present in the layer at a concentrationsuch that a ratio of aluminum atoms to the sum of aluminum atoms plussilicon atoms [Al/(Al+Si)] is in a range from 0.01 to 0.15.
 12. Thearticle of claim 1, wherein the dopant comprises a material selectedfrom the group consisting of aluminum, aluminum oxide, aluminum carbide,aluminum nitride, aluminum boride, and combinations including any of theaforementioned.
 13. The article of claim 1, wherein the dopant comprisesaluminum oxide.
 14. The article of claim 13, wherein the aluminum oxideis present in the layer (104, 105) at a concentration such that a ratioof aluminum atoms to the sum of aluminum atoms plus silicon atoms[Al/(Al+Si)] is in a range from 0.01 to 0.15.
 15. The article of claim13, wherein the aluminum oxide is present in the layer in the form ofnanoparticles.
 16. The article of claim 13, wherein the layer compriseselemental silicon, a silicon alloy, a silicide, or a combinationincluding any of the aforementioned.
 17. The article of claim 1, furthercomprising a top coat disposed over the layer, the top coat comprisingan oxide.
 18. The article of claim 17, wherein the top coat comprisescomprising a silicate, an aluminosilicate, a rare earth silicate,yttria-stabilized zirconia, or a combination including any of theaforementioned.
 19. An article comprising: a silicon-bearing ceramicmatrix composite, wherein the composite comprises a dopant, the dopantcomprising aluminum.
 20. The article of claim 19, wherein the dopantcomprises a material selected from the group consisting of aluminum,aluminum oxide, aluminum carbide, aluminum nitride, aluminum boride, andcombinations including any of the aforementioned.
 21. The article ofclaim 20, wherein the aluminum is present in a doped region of thecomposite at a concentration such that a ratio of aluminum atoms to thesum of aluminum atoms plus silicon atoms [Al/(Al+Si)] is in a range from0.01 to 0.15.
 22. The article of claim 19, wherein the compositecomprises silicon carbide, silicon nitride, or a combination includingany of the aforementioned.
 23. The article of claim 19, furthercomprising at least one coating disposed over the composite.
 24. Thearticle of claim 19, further comprising a bond coat disposed over thecomposite and a top coat disposed over the bond coat.
 25. An articlecomprising: a substrate comprising a silicon-bearing ceramic matrixcomposite; a bond coat disposed over the substrate, the bond coatcomprising elemental silicon, a silicon alloy, a silicide, or acombination including any of the aforementioned, and further comprisingaluminum oxide at a concentration such that a ratio of aluminum atoms tothe sum of aluminum atoms plus silicon atoms [Al/(Al+Si)] in the bondcoat is in a range from 0.01 to 0.15; and a topcoat disposed over thebond coat, the topcoat comprising a silicate, an aluminosilicate, a rareearth silicate, yttria-stabilized zirconia, or a combination includingany of the aforementioned.
 26. An article comprising: a substratecomprising a silicon-bearing ceramic matrix composite; a bond coatdisposed over the substrate, the bond coat comprising silicon, a siliconalloy, a silicide, or combinations including any of the aforementioned,and optionally further comprising a dopant comprising aluminum oxide; anover-coat disposed over the bondcoat, the over-coat comprising silicaand a dopant comprising aluminum oxide; and a top coat disposed over theover-coat, the top coat comprising a silicate, an aluminosilicate, arare earth silicate, yttria-stabilized zirconia, or a combinationincluding any of the aforementioned.
 27. An article comprising: aceramic matrix composite substrate, wherein the composite comprises asilicon-bearing ceramic and a dopant, the dopant comprising aluminum; abond coat disposed over the substrate, where the bond coat compriseselemental silicon, a silicon alloy, a silicide, or combinationsincluding any of the aforementioned; and a coating disposed over thebond coat the coating comprising a silicate, an aluminosilicate, a rareearth silicate, yttria-stabilized zirconia, or a combination includingany of the aforementioned.